The invention relates to a design for constructing an input circuit to receive and process an electrical signal, such as a voltage signal from a voltage source, specifically from a sensor, such as an electrochemical, inductive or optical sensor, where the input circuit has an extremely high input resistance of at least 1011 ohms and is located on a printed circuit board, where an first area of the printed circuit board carrying input circuit components is separated from a second area, which surrounds it or is contiguous to it, by a channel-shaped recess.
Specifically the present invention relates to an input circuit for measurement processing equipment to operate a sensor, specifically an electrochemical sensor for measuring pH. The sensor electrodes constitute a voltage source with an internal resistance of up to 1 gigohm. In order to be able to receive and process these voltages and thus measurement signals from the sensor, a suitable input circuit is required which puts such a small load on the voltage source that the error resulting from the reception and processing of the measurement signal is smaller than the acceptable measurement error. Normally an input resistance is required which is than the internal resistance of the electrode array of the sensor higher by a factor of 1000. The input resistance must be at least 1011 ohms, preferably 5xc3x971011 ohms. The preference is for 1013 ohms and higher.
An input circuit, particularly for recording the measurement signal from a pH-sensitive electrode array, normally comprises a high-resistance operational amplifier, which is wired in the circuit as a buffer (1 amplification) and thus amplifies the measurement signal for further processing of the measurement voltage. If an input circuit of this type is located on conventional printed circuit board material, specifically FR4 material, it operates reliably only under laboratory conditions, because under laboratory conditions (suitable temperature, low relative humidity) hardly any leakage current is drained off over the base material of the printed circuit board and of its surface and the surface of components involved.
However, if the input circuit is exposed to higher relative humidity and temperature under operating conditions (for example, 60xc2x0, 95 percent relative humidity), there is a risk that the reading will be falsified because of current leakage across the lower resistance base material on the circuit card, as a result of penetration of water molecules, as well as from current bridges, for example, across etched-in dirt particles on the surfaces of the input circuit and circuit card components. Without special measures, moisture cannot be prevented from penetrating into the base material of standard circuit cards. Standard circuit cards are constructed on a fiberglass-reinforced epoxy resin base and typically are capable of absorbing moisture. Water molecules can become embedded in the chemical structure of the epoxy resin. Furthermore water molecules can migrate by capillary action to the adjacent surfaces between the epoxy resin and embedded glass fibers into the interior of the printed circuit board.
This problem has been solved until now by the use of ceramic base material (so-called hybrid printed circuit boards) and an encapsulation of the input circuit by means of a high-fill, epoxy-based sealing material. This solution is relatively expensive, however, because of the costly material and is associated with time-intensive manufacturing.
In the applicant""s patent DE 198 10 736 A1, a measurement input for a high-resistance input circuit was proposed, where a section of the printed circuit board assigned to the measurement input is isolated from the other areas of the printed circuit board by a dividing gap passing completely across the printed circuit board. This section is connected to an operational amplifier which, however, is not provided on the isolated section of the printed circuit board. It was possible to produce a high-resistance input circuit of this type under DE 198 10 736 A1 economically by assembling the printed circuit board in automated pick-and-place equipment and subsequently creating an insular section as the measurement input for the circuit. Taking this as a point of departure, the object of the present invention is to effectively counteract the effects of moisture on the input circuit.
In addition, as already mentioned above, the use of a ceramic base material (hybrid printed circuit board) as base material in place of conventional printed circuit board materials (FR4 material) and locating the components of the input circuit on it is already known. By encapsulating the input circuit with a high-fill, epoxy-based sealant it was possible to obtain the requisite input resistances on the input circuit. However, as already mentioned, this solution is expensive and time-intensive in production.
In contrast, the object is to bring about more economical manufacturability
The preceding aspects of the object are met under the invention in a high-resistance input circuit of the generic type by having the channel-shaped recess in the interior of the printed circuit board stop, at least in sections not form a dividing gap passing completely through the printed circuit board, as in DE 198 10 736 A1, and extend directly in the direction of its thickness as far as a moisture-impermeable barrier layer (diffusion dam) which underlies the first area of the printed circuit board and by having the channel-shaped recess and the first area filled and enclosed by a cohesive moisture-impermeable sealing material.
The high-resistance components of the input circuit are therefore placed on the first area, which is made of conventional printed circuit board material, specifically FR-4 printed circuit board material, and overlaid by the moisture-impermeable sealing material. This first printed circuit board area is closed and sealed in a moisture-tight manner to the bottom and to the inside, specifically by the moisture-impermeable barrier layer on one side and laterally by the sealing material which fills the circumferential channel-shaped recess. As a result of the circumferential channel-shaped recess being extended right up to the moisture-impermeable barrier layer, complete sealing of the first printed circuit board area against moisture is achieved. So if any moisture which has found its way into the printed circuit board material penetrates from the inside in the direction of the first area on which the input circuit is located, the moisture is prevented from progressing to the circuit components by the moisture-impermeable barrier layer and by the sealing material filling the channel-shaped recess. The moisture-impermeable barrier coat can be achieved or configured in any way as long as effective blocking action against penetrating moisture is obtained. It has proved to be effective if the moisture-impermeable barrier layer is formed by a metallic layer inside the printed circuit board. This metallic layer is preferably flat, it extends continuously under the first area, that is without any breaks. The primary material of the printed circuit board can be a copper-clad printed circuit board, specifically a multi-layer printed circuit board. With respect to economical manufacturability of the printed circuit board, it proves to be advantageous if it is made from conventional FR-4 material, which has at least one moisture-impermeable barrier layer on the inside.
It has furthermore proved to be quite particularly advantageous if the walls of the printed circuit board adjacent to the channel-shaped recess have been furnished on their part with a moisture-impermeable coating, specifically in the form of a metal plating prior to pouring in the sealing material. This moisture-impervious coating is then in direct contact with the moisture-impervious barrier layer which underlies the first area of the printed circuit board and seals it against moisture. The moisture-impervious sealing material is produced on an epoxy base or on a high-density polyethylene or liquid resin base.